There are many devices in which a magnetic detection circuit is used. Among these, coding devices, from which high precision and accurate operation is required, may be designated as being representative of examples of application. In general, multi-revolution coding devices are conventional as coding devices that have a magnetic detection circuit. In addition to an arrangement for determining the degree of positional displacement within one rotation, these encoders have a rotating magnetic body, which is referred to as a ring magnet for detecting multiple revolutions and is for determining number of revolutions from one revolution and beyond, and a magnetic detection circuit for detecting the change in magnetism of this rotating magnetic body. FIG. 8 illustrates the arrangement of a conventional multi-revolution encoder.
The encoder having the arrangement illustrated in FIG. 8 has: a rotating shaft 12, which is mounted, and therefore coupled, to a rotating body that is the object to be measured; bearings 15, which rotatably support this rotating shaft 12; and a base element 11, on which these bearings 15 are fixed. In order for the shaft of the rotating body to be attached, a through-hole 12a having approximately the same diameter as this shaft is formed in the lower region of rotating shaft 12. A code disc (slotted plate) 14 is mounted above the section on which bearings 15 are supported, and a rotating magnetic body (ring magnet) 16 is mounted above this code disc. A base plate 13, which is fixed to base element 11, is arranged above code disc 14 and faces this code disc. A light-receptor element 19, which is provided with a fixed slot 20, is mounted to base plate 13 at a point opposite to the code (slot) forming region of code disc 14, and electrically connected to circuits on the base plate. A light-generating element 21 is arranged beneath code disc 14 at a position corresponding to the mounting position of light-receptor element 19, so that the light produced by light-generating element 21 may be detected by light-receptor element 19 via the slots of code disc 14 and fixed slot 20. The degree of positional displacement within one revolution is measured, using light-generating element 21, code disc 14, light-receptor element 19, etc.
Rotating magnetic body 16 attached to rotating shaft 12 in the region above base plate 13 is arranged so that its magnetic poles reverse while making one revolution. A magnetic detection element 17, which detects the change in the magnetic poles, is positioned so that it faces the circumferential surface of the rotating magnetic body. A biasing magnet 18 for providing magnetic detection element 17 with a biasing magnetic field is formed on the side of magnetic detection element 17 opposite to the side facing the circumferential surface of the rotating magnetic body. Magnetic detection element 17 and biasing magnet 18 are mechanically and electrically connected to base plate 13.
Magnetoresistive elements may be used as magnetic detection elements. In the case of a multi-revolution detection circuit that uses magnetoresistive elements, a bridge circuit is formed, as illustrated in FIG. 9, for example, by two pairs of magnetoresistive elements, MR1 and MR2, as well as MR3 and MR4, which are arranged at points that are separated from one another by 180 degrees, so as to surround the rotating magnetic body. Differential signals are output via intermediate points n1 and n2. These differential signals are input into a comparator 5, a comparison or reference waveform is produced, and a detection signal f is obtained.
The circuit illustrated in FIG. 9 is a bridge circuit, which is formed between current-source terminal Vcc and grounding terminal GND. A bridge section made up of first magnetic detection element MR1, of which one terminal is connected on the side of the current-source terminal, and third magnetic detection element MR3, as well as an adjustable resistor R3 connected to a terminal of third magnetic detection element MR3, is provided on the one side of the bridge circuit. A bridge section made up of second magnetic detection element MR2, of which one terminal is connected on the side of the current-source terminal, and fourth magnetic detection element MR4, as well as a fixed resistor R4 connected to a terminal of fourth magnetic detection element MR4, is present on the other side of the bridge circuit. Intermediate point n1 between first magnetic detection element MR1 and third magnetic detection element MR3, and intermediate point n2 between second magnetic detection element MR2 and fourth magnetic detection element MR4 are each connected to a differential signal input of comparator CMP. The output of comparator CMP is connected to an output terminal n3.
The behavior of this circuit is explained in more detail. The revolutions of a rotating magnetic body (ring magnet), such as that illustrated in FIG. 4, in which half of the upper surface of a flat disk surface is magnetized as a north pole (and the back surface as a south pole) and the other half is magnetized as a south pole (and the back surface as a north pole), are detected, or the movements of a linear magnetic body (linear magnet), such as that illustrated in FIG. 5, which is arranged in a straight line, are detected. In the explanation below, the behavior in the case of using a rotating magnetic body is explained, but the behavior in the case of a linear magnetic body is approximately the same.
The magnetic field of the rotating magnetic body is initially sensed by magnetoresistive elements MR3 and MR2 and then, after a half revolution of the rotating magnetic body, by magnetoresistive elements MR1 and MR4. In this context, signals e1 and d1, which are illustrated in FIG. 7 and represent differential signals, are produced at intermediate points n1 and n2 of the bridge circuit, which is formed by the individual magnetoresistive elements. Afterwards, a comparison is made and a waveform is produced in comparator CMP, and the signal produced at output terminal n3 is a pulse, such as that illustrated in FIG. 7, which corresponds to one revolution. This pulse signal is used as a multiple-revolution detection signal of the rotary encoder (referred to in the following as “multi-revolution detection signal”).
Although magnetoresistive elements may be expensive, such a bridge circuit requires the use of two pairs of magnetoresistive elements. Therefore, the detection circuit has component costs, which may represent a significant obstacle to reducing the cost of the encoder.
Magnetic detection circuits, which use magnetic detection elements such as magnetoresistive elements, are also described, for example, in Japanese Examined Patent Application Publication No. 2715997 (Japanese Published Patent Application No. 9-5413) and Japanese Published Patent Application No. 11-337368, but in both cases, nothing different is described beyond the fact that two magnetic detection elements, e.g., one pair, are arranged in both or in one of the bridge section that form the bridge circuit. In FIG. 3 of Japanese Published Patent Application No. 11-337368, the number of magnetic detection elements is reduced by one pair, and if only one pair is used in one of the bridge sections that form the bridge circuit, then no differential signals may be obtained in this arrangement.